The row centres on a long-running national industrial dispute between university staff and employers, which was only settled this month when staff accepted a 2pc pay rise.

Members of the University and College Union (UCU) staged three two-hour strikes in January and February, as well as three full-day strikes.

Now the union has written to 26 universities, including the University of East Anglia (UEA) and the Norwich University of the Arts (NUA), which it said are refusing to pay back money they say was unlawfully deducted.

UCU regional official Ben Monks said: “We have written to both the Norwich institutions and told them to pay back the money taken. Their hard-working staff took industrial action in a legitimate attempt to reverse years of declining pay.

The UEA. Picture: Bill Smith

“To be treated so poorly by your employer leaves an incredibly bitter taste in the mouth. At the end of such a protracted, and at times bitter, dispute Norwich’s universities have this opportunity to start to improve relations with their staff.

“We consider any money taken from staff for more than the two hours they were on strike as an unlawful deduction. If the universities do not repay the money we will seek redress through the courts on behalf of our members.”

The union said the case of NUA was “particularly galling” because of vice-chancellor John Last’s 13pc pay rise last year.

A spokesman for NUA said: “The university was well supported by its staff throughout the threatened action and there was minimal action taken. The university is pleased that the national dispute has been resolved and the 2014/15 pay award now agreed.

“NUA policies relating to pay on strike action have been in place for a number of years and are based on guidelines from the Universities and Colleges Employers Association.”

A spokesman for the UEA said: “UEA has been clear from the very outset of the dispute that we do not accept partial performance and our stance has been consistent with the advice given by the Universities and Colleges Employers Association.

“We have just received a letter from UCU asking us to reflect on the decision, which we will consider in due course.”

UCU general secretary Sally Hunt has written to NUA and UEA’s vice chancellors.